High stability, high lubricity, low toxicity, high biodegradability drilling fluid

ABSTRACT

A drilling fluid including at least about 2 wt % olefin, at least about 5 wt % n-paraffins, and at least about 2 wt % isoparaffins, wherein the isoparaffins are substantially wholly terminal monomethyl branched is provided. A method for producing the drilling fluid, and a method for drilling a borehole using the drilling fluid are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional ApplicationSerial No. 60/449,557, filed on Feb. 24, 2003.

FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

REFERENCE TO MICROFICHE APPENDIX

[0003] Not applicable.

FIELD OF THE INVENTION

[0004] The present invention relates to drilling fluids derived from theproducts of a Fischer-Tropsch synthesis. More particularly, theinvention relates to base fluids, and drilling fluids producedtherefrom, having high stability, lubricity, and biodegradability whilealso having low toxicity. The invention further relates to processes ofproducing and using the drilling fluids.

BACKGROUND OF THE INVENTION

[0005] Drilling fluids, or drilling muds, are well known for use in oiland gas well drilling operations. In early applications, crude, dieseland mineral oils were used as base fluids in formulating drillingfluids. Such oil based drilling fluids, however, have unacceptabletoxicity and persistence, or non-biodegradability. Toxicity andpersistence issues are especially critical in offshore drillingoperations. Pseudo-oil based drilling fluids are also well known whereinthe base fluids are fatty acid esters or synthetic hydrocarbons. Suchsynthetic hydrocarbons have included polyalphaolefins, linearalphaolefins, internal olefins, and mixtures thereof as well as linearand branched paraffins and mixtures thereof. Synthetic olefinic andparaffinic base fluids produced by oligomerizing low carbon numberhydrocarbons are well known but are expensive to produce. Similarly,ester base fluids are expensive to produce. 161 The cost of drillingfluids may be reduced by use of Fisher-Tropsch synthesis to produceparaffinic base fluids. Paraffinic drilling fluids, however, generallydo not have adequate low temperature properties for use in offshore andcold weather drilling operations.

[0006] One approach to the low temperature issue incorporatesmulti-methyl branched isoparaffins or isoparaffins with branches ofhigher carbon number than methyl into the paraffinic base fluid. Suchparaffinic base fluids contain C₁₀-C₂₄ n-paraffins and isoparaffins,with an iso to normal ratio ranging from 0.5:1 to 9:1. Greater than 50%of the total isoparaffin content are mono-methyl branched isoparaffinsand about 30% are multi-methyl substituted. A similar approach to thelow temperature issue involves formulation of a drilling fluidcontaining a mixture of n-paraffins and isoparaffins wherein at least90% of the isoparaffins are mono- or poly-methyl branched isomers.Although the use of Fisher-Tropsch synthesis somewhat lowers the cost ofsuch paraffinic drilling fluids, the necessity of hydrocracking andhydroisomerization adds to the overall cost of the drilling fluid.Moreover, to sufficiently improve the low temperature properties of theparaffinic base fluids, pour point depressant additives may be required,further increasing the cost of the drilling fluid.

[0007] In another known drilling fluid a base fluid of a mixture oflinear and branched alphaolefins with the linear to branch ratio rangingfrom 1:1 to 5:1 is used. In this known drilling fluid the olefins may beproduced from high temperature Fischer-Tropsch synthesis. Hightemperature Fischer-Tropsch synthesis, however, results in primarilyinternal branching of the branched hydrocarbons. Thus, such drillingfluids are not wholly biodegradable resulting in increased persistence.Moreover, because of the reactivity of olefins, drilling fluids composedof olefins are easily oxidizable and, therefore, must be stored under anitrogen blanket to provide a reasonable shelf life. Even with nitrogenpadding, olefinic drilling fluids cannot be stored for periods as longas paraffinic drilling fluids. Therefore, the cost of using olefinicdrilling fluids is increased by both its special storage requirementsand loss of oxidized product.

[0008] Therefore, there remains a need for a further improved lower costdrilling fluid having oxidative stability, high lubricity, low toxicity,improved low temperature properties, and high biodegradability.

SUMMARY OF THE INVENTION

[0009] In one embodiment of the invention, a base fluid which may beused for drilling fluid formulation is provided. The base fluid includesat least about 5 wt % olefins, at least about 5 wt % n-paraffins andbetween about 2 and 50 wt % branched paraffins wherein substantially allof the branch groups are monomethyl and wherein the ratio of terminalmonomethyl branching to internal monomethyl branching is at least about1:1.5.

[0010] In other embodiments of the invention a drilling fluid isprovided wherein the drilling fluid contains between about 2 and about90 wt % olefins, between about 5 and about 90 wt % n-paraffins andbetween about 0 and about 10 wt % oxygenates.

[0011] Yet another embodiment of the invention provides a process forproducing a drilling from a light Fischer-Tropsch liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] None.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0013] The term “C_(x)”, where x is a number greater than zero, refersto a hydrocarbon compound having predominantly a carbon number of x. Asused herein, the term C_(x) may be modified by reference to a particularspecies of hydrocarbons, such as, for example, C₅ olefins. In suchinstance, the term means an olefin stream comprised predominantly ofpentenes but which may have impurity amounts, i.e. less than about 10%,of olefins having other carbon numbers such as hexene, heptene, propene,or butene. Similarly, the term “C_(x+)” refers to a stream wherein thehydrocarbons are predominantly those having a hydrocarbon number of x orgreater but which may also contain impurity levels of hydrocarbonshaving a carbon number of less than x. For example, the term C₁₅₊meanshydrocarbons having a carbon number of 15 or greater but which maycontain impurity levels of hydrocarbons having carbon numbers of lessthan 15. The term “C_(x)-C_(y)”, where x and y are numbers greater thanzero, refers to a mixture of hydrocarbon compounds wherein thepredominant component hydrocarbons, collectively about 90% or greater byweight, have carbon numbers between x and y inclusive. For example, theterm C₅-C₉ hydrocarbons means a mixture of hydrocarbon compounds whichis predominantly comprised of hydrocarbons having carbon numbers between5 and 9 inclusive, but may also include impurity level quantities ofhydrocarbons having other carbon numbers.

[0014] As used herein the term “high lubricity” means having a wear scarof average diameter of about ≦0.46 mm at 60° C. tested in accordancewith ASTM Standard D-6079-02 entitled “Standard Test Method forEvaluating Lubricity of Diesel Fuels by the High-Frequency ReciprocatingRig.” The terms “high stability” and “high oxidative stability” meanhaving a total solids ≦1.5 mg/100 ml tested in accordance with ASTMStandard D-22-74-Ola entitled “Standard Test Method for OxidationStability of Distillate Fuel Oil (Accelerated Method).” Note that thesemethods are being applied herein to the analysis and characterization ofsynthetic products although the standards refer expressly to petroleumderived products.

[0015] Unless otherwise specified, all quantities, percentages andratios herein are by weight.

[0016] Embodiments of the drilling fluid of the invention contain a basefluid having from about 5 to about 90 wt % linear alpha- and internalolefins, from about 5 to about 20 wt % isoparaffins, from about 5 toabout 90 wt % n-paraffins and from about 0 to about 10 wt % oxygenates.To fully realize the lower cost potential of the invention, the basefluid for the drilling fluid of such embodiments may be obtained from aFischer-Tropsch synthesis using synthesis gas as a feed stock. Moreover,production of the drilling fluid from a base fluid produced by theFischer-Tropsch synthesis and subsequent processing as described hereinis desirable as it results in a product having the desirable olefin andparaffin contents. However, the base fluid composition may be otherwiseproduced while yet retaining the remaining advantages of the invention.Thus, some embodiments of the base and drilling fluids of the inventionare not produced by Fischer-Tropsch synthesis.

[0017] Three basic techniques may be employed for producing a synthesisgas, or syngas, which is used as the starting material of aFischer-Tropsch reaction. These include oxidation, reforming andautothermal reforming. As an example, a Fischer-Tropsch conversionsystem for converting hydrocarbon gases to liquid or solid hydrocarbonproducts using autothermal reforming includes a synthesis gas unit,which includes a synthesis gas reactor in the form of an autothermalreforming reactor (ATR) containing a reforming catalyst, such as anickel-containing catalyst. A stream of light hydrocarbons to beconverted, which may include natural gas, is introduced into the reactoralong with oxygen (O₂). The oxygen may be provided from compressed airor other compressed oxygen-containing gas, or may be a pure oxygenstream. The ATR reaction may be adiabatic, with no heat being added orremoved from the reactor other than from the feeds and the heat ofreaction. The reaction is carried out under sub-stoichiometricconditions whereby the oxygen/steam/gas mixture is converted to syngas.Examples of Fischer-Tropsch systems are described in U.S. Pat. Nos.4,973,453; 5,733,941; 5,861,441; 6,130,259, 6,169,120 and 6,172,124, thedisclosures of which are herein incorporated by reference.

[0018] The Fischer-Tropsch reaction for converting syngas, which iscomposed primarily of carbon monoxide (CO) and hydrogen gas (H₂) may becharacterized by the following general reaction:

2nH ₂ +nCO →(—CH₂—)_(n) +nH ₂ O  (1)

[0019] Non-reactive components, such as nitrogen, may also be includedor mixed with the syngas. This may occur in those instances where air orsome other non-pure oxygen source is used during the syngas formation.

[0020] The syngas is delivered to a synthesis unit, which includes aFischer-Tropsch reactor (FTR) containing a Fischer-Tropsch catalyst.Numerous Fischer-Tropsch catalysts may be used in carrying out thereaction. These include cobalt, iron, ruthenium as well as other GroupVIIIB transition metals or combinations of such metals, to prepare bothsaturated and unsaturated hydrocarbons. For purposes of this invention,a non-iron catalyst may be used. The F-T catalyst may include a support,such as a metal-oxide support, including silica, alumina, silica-aluminaor titanium oxides. For the purposes of this reaction, a Co catalyst ontransition alumina with a surface area of approximately 100-200 m²/g isused in the form of spheres of 50-150 μm in diameter. The Coconcentration on the support may also be 15-30%. Certain catalystpromoters and stabilizers may be used. The stabilizers include Group IIAor Group IIIB metals, while the promoters may include elements fromGroup VIII or Group VIIB. The Fischer-Tropsch catalyst and reactionconditions may be selected to be optimal for desired reaction products,such as for hydrocarbons of certain chain lengths or number of carbonatoms. Any of the following reactor configurations may be employed forFischer-Tropsch synthesis: fixed bed, slurry reactor, ebullating bed,fluidizing bed, or continuously stirred tank reactor (CSTR). For thepurposes of this reaction, a slurry bed reactor is used. The FTR may beoperated at a pressure of 100 to 500 psia and a temperature of 375° F.to 500° C. The reactor gas hourly space velocity (“GHSV”) may be from1000 to 8000 hr⁻¹. Syngas useful in producing a Fischer-Tropsch productuseful in the invention may contain gaseous hydrocarbons, hydrogen,carbon monoxide and nitrogen with H₂/CO ratios from about 1.8 to about2.4. The hydrocarbon products derived from the Fischer-Tropsch reactionmay range from methane (CH₄) to high molecular weight paraffinic waxescontaining more than 100 carbon atoms. An overhead product stream isrecovered from the FTR and may be separated into tail gas and lightFischer-Tropsch liquid (“LFTL”) products in a cold separator. The LFTLstream may then be distilled to yield a hydrocarbon product of primarilyC₁₃-C₂₂ olefins and paraffins.

[0021] The C₁₃-C₂₂ distillation cut may be used directly as the basefluid. Low levels of C¹³⁻hydrocarbons may be present in the C₁₃-C₂₂ cutprovided such levels do not cause the drilling fluid to exceed currentregulatory limits. C¹³⁻hydrocarbons may be present in amounts betweenabout 0.01 and 10 wt %. The presence of such small amounts of C¹³⁻hydrocarbons permits incorporation of a wider distribution ofC₁₃₊hydrocarbons, particularly C₁₆₊hydrocarbons, for improved lubricity.The hydrocarbon number distribution is such as to result in a drillingfluid pour point below about 10° C.

[0022] Embodiments of the base fluid contains from about 5 to about 90wt % linear alpha- and internal olefins. The olefin content may providethe mixture with lower pour-point, better surface activity, betterlubricity and better adherence to metal. When the base fluid is producedfrom the Fischer-Tropsch synthesis with the appropriate Fischer-Tropschcatalyst and operating conditions, the Fischer-Tropsch product will haveapproximately 5% alpha and internal olefin content. Depending upon thereaction conditions of the FTR and catalyst used in the Fischer-Tropschreaction, it may be necessary to concentrate the olefin content toachieve the higher percentages of olefins in the base fluid.Concentration of olefins may be undertaken, for example, by one or moreof the following known techniques: (1) molecular sieve separation ofolefins and paraffins, such as UOP's OLEX® process, and (2) distillationof paraffins away from individual C_(X) cuts.

[0023] The base fluid may contain between about 5 to about 95 wt %paraffins. Of the total paraffin content from about 3 to about 20 wt %are isoparaffins. Substantially all of the isoparaffins are terminalmonomethyl species. As used herein, the terminal species include 2- and3-methyl branched. The presence of monomethyl isoparaffins improves lowtemperature properties, such as pour point, as well as lubricity andviscosity. Moreover, because the isoparaffins are predominantlyterminally branched, the paraffin content of the base fluid issubstantially wholly biodegradable. Using the Fischer-Tropsch synthesisdescribed herein, about 5 wt % terminal methyl branched paraffins areproduced in the LFTL.

[0024] Concentration of isoparaffins may be increased by one or more ofthe following techniques: (1) molecular sieve separation of linear andbranched paraffins, such as UOP's OLEX® process, and (2) isomericdistillation of isoparaffin.

[0025] Embodiments of the base fluid contain between about 0 and about10 wt % oxygenates. The oxygenates are principally primary alcohols.Other oxygenates, including aldehydes, ketones, carboxylic acids andesters and di-esters of carboxylic acids are also present in smallamounts. Oxygenate content in the C₁₃-C₂₂ cut of the Fischer-Tropschreaction product ranges from between about 0.5 to about 5.0 wt %. Lowlevels of oxygenates in the drilling fluid from between about 0 andabout 10 wt % provide improved lubricity. Moreover, oxygenates mayassist in emulsification in invert drilling muds.

[0026] Oxygenate control may be used on the C₁₃-C₂₂ cut of theFischer-Tropsch product stream. In one embodiment of the invention, abase fluid is produced by vaporizing a product stream and passing thevaporized product over an activated alumina catalysts to dehydratealcohols to corresponding olefins. The conversion of the alcohol contentof the product stream occurs according to the following reaction:

CH₃—(CH₂)_(x)—CH₂—CH₂OH→CH₃—(CH₂)_(x)—CH═CH₂+H₂O  (2)

[0027] For example, the LFTL, C₁₃-C₂₂ or other product stream may bevaporized at a temperature from about 400° F. to about 800° F. and thenpassed over one or more dehydration beds containing activated treatedalumina or silica-alumina. In some embodiments, the dehydration beds arepacked beds. Essentially all of the primary and internal alcoholspresent in the vaporized stream are dehydrated to their correspondingolefins, with conversion rates of at least 95%.

[0028] Dehydration reaction temperature may range from between about400° and 800° F. The vaporized feed for the dehydration unit may besuperheated prior to being fed into the dehydration beds oralternatively, may be heated within packed beds. The LHSV of packed bedsmay range from about 0.10 hr⁻¹ to about 2.0 hr⁻¹. Reaction pressure maybe maintained by the pressure of the accumulator and must be such tovaporize all of the dehydration feed. Typically, the pressure may rangefrom between about 0 psia to about 100 psig.

[0029] In an alternative embodiment, a moving bed of alumina orsilica-alumina catalyst may be used. Fluidized beds, slurry beds orebullating beds may be used with continuous batch or semi-batch catalystremoval and regeneration. The catalyst may be removed by one of thesemethods and regenerated by passing a mixture of nitrogen and oxygen orair at elevated temperatures over the catalyst.

[0030] Depending upon the alumina used, some of the olefins present orproduced in the dehydration beds may also be isomerized to internalolefins. Alumina catalysts useful for the dehydration of alcohols areknown and include, for example, gamma-alumina, theta-alumina, pacifiedalumina, and activated alumina. High surface area aluminas areparticularly useful in the invention and include those aluminas having asurface area of about 100 m²/gm or greater. Commercially availablealumina useful in the integrated Fischer-Tropsch process include, forexample, S-400, which has a surface area of about 335 m²/gm, and DD-470,which has a surface area of about 375 m²/gm. S-400 ad DD-470 are aluminacatalysts made and sold by Alcoa. Alumina catalysts for use in theintegrated Fischer-Tropsch process generally contain at least about 90wt % Al₂O₃, oxides of silicon and iron present in amounts of less thanabout 0 wt %, and oxides of sodium present in an amount of less thanabout 1 wt %. The alumina catalysts are generally supplied assubstantially spherical particles having diameter from about ⅛ to about¼ inch.

[0031] In another embodiment of the invention, molecular sieve orzeolitic molecular sieve forms of the alumina or silica-aluminacatalysts may be used. For example, silico alumino phosphate (“SAPO”)molecular sieves may be used in the packed beds. SAPO molecular sievescontain a 3-dimensional microporous crystal structure having 8, 10, or12 membered ring structures. The ring structures can have an averagepore size ranging from between about 3.5 angstroms to about 15angstroms. Other silica-containing zeolitic molecular sieve catalysts,such as ZSM-5, may be used in the dehydration bed.

[0032] Following dehydration, the aqueous and organic phases may beseparated. Such dehydration process may further be used to increase theolefin content of the product stream to be used to produce the basefluid.

[0033] Other methods of oxygenate control include, for example, reactionof the alcohol content of the C₁₃-C₂₂ cut of a Fischer-Tropsch productstream with maleic or succinic anhydride or with a carboxylic acid, suchas formic acid, acetic acid, or other acids. The carboxylic acid estersmay be retained in the stream as they are excellent lubricants which arealso highly biodegradable. Both the lubricity and the biodegradabilityof the base fluid may be improved by converting at least a portion ofthe alcoholic oxygenate content to carboxylic acid esters.

[0034] The drilling fluid may optionally include one or more surfactants(e.g., emulsifiers, wetting agents), viscosifiers, weighting agents,fluid loss control agents, and proppants. Because the drilling fluidshould be non-toxic, these optional ingredients, like the base fluid,are preferably also non-toxic. Acceptable emulsifiers include, but arenot limited to, fatty acids, and fatty acid derivatives includingamido-amines, polyamides, polyamines, esters, imidaxiolines, andalcohols. Typical wetting agents include, but are not limited to,lecithin, fatty acids, crude tall oil, oxidized crude tall oil, organicphosphate esters, modified imidazolines, modified amidoamines, alkylaromatic sulfates, alkyl aromatic sulfonates, and organic esters ofpolyhydric alcohols. Exemplary weighting agents include, but are notlimited to, barite, iron oxide, gelana, siderite, calcium oxide, andcalcium carbonate. Acceptable proppants include sand, gravel, and nutshells. Exemplary viscosifiers include, but are not limited to,organophilic clays, non-organophilic clays, oil soluble polymers,polyamide resins, and polycarboxylic acids and soaps. Where additivesare used in the drilling fluid, the base fluid constitutes from about 25to about 85 volume percent of the total drilling fluid. Illustrativefluid loss control agents include, but are not limited to, asphaltics(e.g., asphaltenes and sulfonated asphaltenes), modified lignites, andpolymers, such as polystyrene, polybutadiene, polyethylene,polypropylene, polybutylene, polyisoprene, natural rubber, and butylrubber.

[0035] The following examples illustrate, but are not intended to limit,the invention.

EXAMPLE 1

[0036] A pilot installation consisting of two distillation columns wasused to produce C₆₋₁₀ naphtha, C₁₀₋₁₃ light kerosene, and C₁₃₋₂₀₊drilling fluid feedstock streams. The columns were fed approximately3400 g/hr of light Fischer Tropsch liquid (LFTL). The LFTL feed hadapproximately the composition shown in Table 1: TABLE 1 Carbon # % bywt.  4 <0.1  5 0.01  6 0.3  7 1.0  8 2.9  9 5.9 10 8.1 11 9.2 12 9.5 139.2 14 8.4 15 7.9 16 7.1 17 6.2 18 5.4 19 4.6 20 3.7 21 3.0 22 2.3 231.7 24 1.2   25+ 2.6 Total 100.00

[0037] The LFTL feed was introduced into a first packed distillationcolumn and C₁₃ and lighter materials were distilled overhead. The firstdistillation column was operated at the following conditions: 10 psig,480° F. feed preheat temperature, 407° F. overhead temperature, 528° F.bottoms temperature. The first distillation column had approximately 98inches of Sulzer Mellapack 750Y packing. The bottoms of the firstdistillation column constituted a C₁₃₋₂₀₊hydrocarbon fraction, thecomposition of which is shown in Table 2. The overhead stream of thefirst distillation column was fed into a second packed distillationcolumn operated at 12 psig, 370° F. overhead temperature and 437° F.bottoms temperature. The second distillation column had about 28 inchesof Sulzer EX packing. The bottoms of the second column TABLE 2 Totaln-paraffins, isoparaffins, olefins and alcohols C₁₁−: Mass % 0.97 C₁₂:Mass % 1.77 C₁₃: Mass % 11.43 C₁₄: Mass % 13.68 C₁₅: Mass % 12.35 C₁₆:Mass % 10.96 C₁₇: Mass % 9.06 C₁₈: Mass % 7.84 C₁₉: Mass % 6.79 C₂₀:Mass % 7.04 C₂₁: Mass % 5.66 C₂₂: Mass % 4.63 C₂₃₊: Mass % 7.83 100.00

EXAMPLE 2

[0038] The C₁₃₋₂₀ and stream from Example 1 was fed via a syringe pumpand mixed with 20 cc/min of nitrogen. The gas/liquid mixture wasintroduced upflow into a vessel packed with stainless steel meshsaddles, where the liquid was vaporized and superheated to reactiontemperature of 675° F. The vaporized feed was fed upflow into a reactorpacked with ⅛ Alcoa S-400 alumina catalyst and suspended in a heatedsandbath. The sandbath was maintained at the reaction temperature andebulated by air. Reactor LHSV was maintained at about 0.26 hr⁻¹. Theoutlet pressure was maintained at about 5 psig. The reaction productcomposition is shown in Table 3. TABLE 4 Sample Reference Number FEED BPRODUCT D TOTAL N-PARAFFIN mass % 82.46 82.87 ALPHA OLEFIN mass % 2.263.48 INTERNAL OLEFIN mass % 2.75 3.68 BRANCHED PARAFFIN mass % 10.109.97 ALCOHOL mass % 2.45 0.00 100.00 100.00

EXAMPLE 3

[0039] An LFTL feed, substantially having the composition shown in Table1, was hydrotreated at reactor conditions of 800 psig and 550° to 590°F. The resulting hydrotreated stream was distilled under conditions asdescribed in Example 1 forming a hydrotreated analog of the C₁₀₋₁₃ lightkerosene product. This hydrotreated light kerosene material was analyzedon a Hewlett Packard Series II gas chromatograph with 60 m RTX 1 columnwith 0.32 mm diameter and 3 micron film thickness. The isomer content ofthis product is shown in Table 4. TABLE 5 Component Wt. % nC₉- 0.02 2-and 3-monomethyl C₁₀ 0.20 4- and higher - monomethyl C₁₀ 0.03 nC₁₀ 22.222- and 3-monomethyl C₁₁ 1.19 4- and higher - monomethyl C₁₁ 0.42 nC₁₁27.93 2- and 3-monomethyl C₁₂ 1.09 4- and higher - monomethyl C₁₂ 0.50nC₁₂ 24.96 2- and 3-monomethyl C₁₃ 0.92 4- and higher - monomethyl C₁₃0.48 nC₁₃ 18.99 2- and 3-monomethyl C₁₄ 0.11 4- and higher - monomethylC₁₄ 0.13 nC₁₄ 0.41 Total normal hydrocarbon 94.54 Total 2- and3-monomethyl 3.51 substituted hydrocarbons Total 4- and higher - 1.55monomethyl substituted hydrocarbons Total monomethyl hydrocarbons 99.61Others 0.39

[0040] While the foregoing describes preferred embodiments of theinvention, it is apparent that a number of changes and variations arewithin the scope and spirit of the invention.

What is claimed is:
 1. A base fluid comprising: at least about 5 wt %olefins; at least about 5 wt % n-paraffins; and between about 2 and 50wt % branched paraffins wherein substantially all of the branch groupsare monomethyl and wherein the ratio of terminal monomethyl branching tointernal monomethyl branching is at least about 1:1.5.
 2. The base fluidof claim 1 wherein the ratio of terminal monomethyl branching tointernal monomethyl branching is at least about 1:1.
 3. The base fluidof claim 1 wherein the n-paraffins are present in an amount of at leastabout 20 wt % and wherein the ratio of terminal monomethyl branching tointernal monomethyl branching is at least about 1.5:1.
 4. The base fluidof claim 1 wherein the n-paraffins are present in an amount of at leastabout 40 wt % and wherein the ratio of terminal monomethyl branching tointernal monomethyl is at least about 2:1.
 5. The base fluid of claim 1wherein the base fluid is a product of a Fischer-Tropsch reaction. 6.The base fluid of claim 5 wherein the Fischer-Tropsch reactionincorporates feed syngas having 10-60% N₂.
 7. A drilling fluidcomprising: the base fluid of claim
 1. 8. The drilling fluid of claim 7further comprising: at least one additive selected from the group ofsurfactants, viscosifiers, weighting agents, fluid loss control agentsand proppants.
 9. A drilling fluid comprising: from about 2 to about 90wt % olefins; from about 2 to about 50 wt % isoparaffins; wherein theisoparaffins are substantially terminal monomethyl branched. from about5 to about 90 wt % n-paraffins; and from about 0 to about 10 wt %oxygenates.
 10. The drilling fluid of claim 9 wherein the olefins arepresent in an amount of from about 7 to about 10 wt %.
 11. The drillingfluid of claim 9 wherein the isoparaffins are present in an amount offrom about 3 to about 15 wt %.
 12. The drilling fluid of claim 9 whereinthe n-paraffins are present in an amount of from about 65 to about 90 wt%.
 13. The drilling fluid of claim 9 wherein the oxygenates are presentin an amount of from about 0 to about 5 wt %.
 14. The drilling fluid ofclaim 9 wherein the base fluid is a product of Fischer-Tropsch reactionon a synthesis gas.
 15. The drilling fluid of claim 14 wherein theFischer-Tropsch reaction incorporates feed syngas having 10-60% N₂. 16.The drilling fluid of clam 14 wherein the synthesis gas is produced byautothermal reformation.
 17. The drilling fluid of claim 16 wherein theautothermal reformation occurs in the presence of air.
 18. The drillingfluid of claim 16 wherein the autothermal reformation occurs in thepresence of 10-60% N₂.
 19. The drilling fluid of claim 9 furthercomprising: at least one additive selected from the group ofsurfactants, viscosifiers, weighting agents, fluid loss control agentsand proppants.
 20. The drilling fluid of claim 9 wherein the olefinsare: from about 7 to about 10 wt %; the isoparaffins are from about 2 toabout 15 wt %; wherein the isoparaffins are substantially terminalmonomethyl branched. The n-paraffins are from about 65 to about 90 wt %;and the oxygenates are from about 0 to about 5 wt %.
 21. The drillingfluid of claim 20 wherein the drilling fluid is a product of aFischer-Tropsch reaction.
 22. The drilling fluid of claim 20 furthercomprising: at least one additive selected from the group ofsurfactants, viscosifiers, weighting agents, fluid loss control agentsand proppants.
 23. The drilling fluid of claim 20 wherein the base fluidcomprises from about 25 to about 85 volume % of the drilling fluid. 24.The drilling fluid of claim 23 wherein the base fluid comprises fromabout 25 to about 85 volume % of the drilling fluid.
 25. The drillingfluid of claim 22 wherein the Fischer-Tropsch reaction incorporates feedsyngas having 10-60% N₂.
 26. The drilling fluid of claim 23 wherein thefeed syngas is produced by autothermal reformation in the presence ofair.
 27. A process for producing a drilling fluid comprising the stepsof: (a) producing a light Fischer-Tropsch liquid; (b) distilling thelight Fischer-Tropsch liquid to obtain a C₁₃-C₂₀₊ product havingC₁₃-C₂₀₊ hydrocarbons and oxygenates. (c) dehydrating all or a part ofthe alcohols in the C₁₃-C₂₀₊ product by passing the C₁₃-C₂₀₊ productover an activated alumina catalyst to produce a dehydrated product; (d)recovering the dehydrated product; and (e) separating the aqueous andorganic phases of the dehydrated product.
 28. The process of claim 27further comprising the step of: (f) adding one or more additive selectedfrom the group of surfactants, viscosifiers, weighting agents, fluidloss control agents and proppants to the organic phase of the dehydratedproduct.
 29. The process of claim 27 further comprising the step of (b₁)vaporizing the C₁₃-C₂₀₊ product before step (c) and after step (b). 30.The process of claim 29 wherein the dehydrated product from step (c) isin the gaseous state and step (d) further includes condensing thedehydrated product.
 31. The process of claim 30 wherein the heat fromcondensing the dehydrated product is recycled to at least partiallyvaporize the C₁₃-C₂₀₊ product in step (b₁).
 32. The process of claim 27wherein the light Fischer-Tropsch liquid is produced from a feed syngashaving 10-60% N₂.
 33. The process of claim 27 wherein the feed syngas isproduced by autothermal reformation in the presence of air.
 34. Theprocess of claim 27 wherein a C₁₄-C₁₈ product is obtained in step (b)and dehydrated in step (c).
 35. A method of drilling a borehole in asubterranean formation comprising the steps of: (a) rotating a drill bitat the bottom of the borehole; (b) introducing a drilling fluid into theborehole wherein the drilling fluid comprises a base fluid comprising:from about 5 to about 90 wt % olefins; from about 2 to about 50 wt %isoparaffins; wherein the isoparaffins are substantially terminalmonomethyl branched; from about 5 to about 90 wt % n-paraffins; and fromabout 0 to about 10 wt % oxygenates.
 36. The process of claim 35 whereinthe drilling fluid comprises: from about 7 to about 10 wt % olefins;from about 2 to about 15 wt % isoparaffins; wherein the isoparaffins aresubstantially terminal monomethyl branched; from about 65 to about 90 wt% n-paraffins; and from about 0 to about 5 wt % oxygenates.
 37. Theprocess of claim 35 wherein the base fluid is derived from aFischer-Tropsch reaction.
 38. The process of claim 37 wherein theFischer-Tropsch reaction incorporates feed syngas having 10-60% N₂. 39.The process of claim 37 wherein the feed syngas is produced byautothermal reformation in the presence of air.